1. Field of the Invention
The present invention relates to a communication system that performs bulk data communication between information apparatuses, and, more particularly to a communication system that performs data communication without interference with other communication systems between information apparatuses using an electrostatic field (a quasi-electrostatic field) or an induction field.
More specifically, the present invention relates to a communication system that realizes bulk transmission by electric-field-coupling using radio-frequency (hereinafter referred to RF) signals, and, more particularly to a communication system that extends a distance between electrodes of a transmitter and a receiver to be field-coupled and makes an arrangement between the transmitter and the receiver and designs of communication apparatuses flexible.
2. Description of the Related Art
Recently, when data is transferred between small-sized information apparatuses, for example, when data of images, music, and the like are exchanged between personal computers, a radio interface is often used instead of a method of transferring data via data communication performed by mutually connecting the apparatuses with a general-purpose cable such as an AV (Audio Visual) cable or a USB (Universal Serial Bus) cable or via a medium such as a memory card. When the radio interface is used, it is unnecessary to reattach a connector and draw around a cable every time data transfer is performed and convenience for a user is high. Many information apparatuses implemented with various cableless communication functions are also placed on the market.
As a method of performing data transfer between small-sized apparatuses without using a cable, radio wave communication schemes for transmitting and receiving radio signals using antennas such as wireless LAN (Local Area Network) communication and Bluetooth (registered trademark) communication represented by IEEE802.11 are developed. For example, a portable image recording apparatus is proposed in which an antenna is built in a position not covered by a hand that grips a grip section and, since the built-in antenna is not covered by the hand and correct image data is received, even if an antenna for radio communication is arranged in the apparatus, a characteristic inherent in the antenna is directly displayed (see, for example, JP-A-2006-106612).
Most of the radio communication systems of the past adopt the radio wave communication scheme and propagates a signal using a radiation-electric-field generated when an electric current is fed to an aerial (an antenna). In this case, since a transmitter emits a radio wave regardless of whether there is a communication partner, the transmitter is likely to be a generation source of a radio wave interfering with a communication system near the transmitter. Since an antenna of a receiver receives not only a desired wave from the transmitter but also radio waves arriving from a distance, the receiver is susceptible to an interference radio wave around the receiver. The interference ratio wave causes the deterioration in reception sensitivity. When there are plural communication partners, it is necessary to perform complicated setting in order to select a desired communication partner out of the plural communication partners. For example, when plural pairs of radios perform radio communication in a narrow range, each of the pairs needs perform division multiplexing such as frequency selection to communicate with each other in order to prevent interference among the pairs. Further, since the transmitter and the receiver may be unable to communicate with each other when directions of polarization of radio waves are orthogonal, polarization directions of antennas need to be the same between the transmitter and the receiver.
For example, considering a non-contact data communication system in a very short distance such as several millimeters to several centimeters, it is preferable that the transmitter and the receiver are firmly coupled in a short distance and, on the other hand, for prevention of interference with other systems, signals do not reach a long distance. It is desirable that apparatuses that perform data communication are coupled without depending on postures (directions) when the apparatuses are places in a short distance, i.e., the apparatuses have not directivity. When bulk data communication is performed, it is desirable that wideband communication is possible.
As the radio communication, other than the radio wave communication performed in which the radiation-electric-field is used, there are communication schemes in which an electrostatic field, an induction field, and the like are used. For example, in the existing non-contact communication system mainly used for RFID (Radio Frequency Identification), an electric-field-coupling scheme and an electromagnetic induction scheme are applied. The electrostatic field and the induction field are inversely proportional to the cube and the square of distances from generation sources thereof, respectively. Therefore, in the non-contact communication system of this type, since a transmission signal steeply attenuates according to a distance, a coupling relation is not formed when no communication partner is present nearby. Thus, the communication system does not interfere with other communication systems. Even if a radio wave arrives from a distance, since an electric-field-coupling antenna (coupler) (hereinafter referred to as EFC antenna) does not receive the radio wave, the communication system is prevented from being interfered by other communication systems.
For example, an RFID tag system is proposed in which, a set of plural communication supporting bodies arranged to place RFID tags among the communication supporting bodies are formed and the RFID tags attached to plural commodities are arranged to be placed among the communication supporting bodies, whereby, even in a state in which the RFID tags overlap each other, it is possible to stably read and write information (see, for example, JP-A-2006-60283).
A data communication apparatus is proposed which includes an apparatus main body and wearing means for wearing this apparatus main body, includes an antenna coil and data communication means for performing data communication in a non-contact manner with an external communication apparatus via this antenna coil, has the antenna coil and the data communication means arranged in an outer caser provided in an upper part of the apparatus main body, and uses an induction magnetic field (see, for example, JP-2004-214879).
A cellular phone is proposed in which an antenna coil for performing data communication with an external apparatus is mounted on a memory card inserted in a portable information apparatus and an antenna coil for RFID is arranged on the outer side of a memory card slot of the portable information apparatus to secure a communication distance without spoiling portability in the RFID (see, for example, JP-A-2005-18671).
The RFID system of the past that uses an electrostatic field or an induction field has a low communication speed because a low-frequency signal is used and, therefore, is unsuitable for bulk data transmission. To cope with this problem, the inventors consider that it is possible to perform bulk transmission by transmitting an RF signal according to electric-field-coupling.
However, whereas the field intensity of a radiation-electric-field gently attenuates in inverse proportion to a distance, the field intensity of the electrostatic field and the field intensity of the induction field steeply attenuate in inverse proportion to the square and the cube of the distance, respectively, i.e., attenuation of a signal due to a communication distance is large. Since a propagation loss occurs according to the length of a propagation distance with respect to a wavelength, when an RF signal is propagated by electric-field-coupling, a problem of a propagation loss corresponding to an inter-electrode distance is conspicuous. Therefore, it is necessary to bring coupling electrodes of a transmitter and a receiver into close contact with each other as much as possible. To bring the coupling electrodes sufficiently close to each other, it is necessary to perform fine positioning between the electrodes and positions of the electrodes have to be held during data communication. Thus, convenience of use for a user is low. Further, it is difficult for the transmitter and the receiver to directly communicate with each other in a long distance.
Moreover, when a coupling electrode is mounted in a housing of a communication apparatus, it is necessary to arrange the electrode on the outer side of the housing as much as possible to reduce the inter-electrode distance during data communication. Thus, a degree of freedom of layout in designing the housing is limited. It goes without saying that it is desirable that the coupling electrode is small and low cost.